The use of polymeric prostheses and biomedical mouldings has grown rapidly in recent times. Such mouldings may be used for contact lenses or for specific ophthalmic purposes. For example, they may be used for intraocular lenses and eye bandages. They may also be used for surgical mouldings such as heart valves and artificial arteries. Other applications include wound dressings, biomedical adhesives and tissue scaffolds. Use in drug delivery is a further application.
Diseases of the lens material of the eye are often in the form of cataracts. The ideal cataract procedure is considered to be one where the lens capsule bag is maintained with the cataractous lens material removed through a small opening in the capsule. The residual lens epithelial cells are removed chemically and/or with ultrasound or lasers. A biocompatible material with appropriate optical clarity, refractive index and mechanical properties is inserted into the capsular bag to restore the qualities of the original crystalline lens. The desired refractive index is 1.41.
There have been recent advances in methods of inserting intraocular lens. For example, U.S. Pat. No. 5,772,667 assigned to Pharmacia Lovision Inc, discloses a novel intraocular lens injector. This device compresses an intraocular lens by rolling the lens into a tight spiral. The device then injects the compressed lens through a relatively small incision in the eye, approximately 2-3 millimetres in length, resulting from a phacoemulsification procedure. The intraocular lens is inserted into a receiving channel of the injector device in an uncompressed state and is urged into a cylindrical passageway. As the intraocular lens advances into the cylindrical passageway, the lens rolls upon itself into a tightly rolled spiral within the confines of the cylindrical passageway. An insertion rod is then inserted into an open end of the cylindrical passageway and advances the compressed lens down the passageway. As the lens exits the passageway and enters the eye, the lens will expand back to its uncompressed state.
To avoid the need for such injection devices, it has been proposed that intraocular lenses be formed in situ after being injected as a liquid flowable form into the lens capsule bag. However, while this concept is attractive in that smaller incisions would be required, it raises further difficulties in that further chemical reactions are required to cure the injectable material and these are required to be not harmful to the patient. It is also a requirement that the chemical reactions can take place over a relatively short time under mild reaction conditions. It is desirable that the reaction is also not significantly inhibited by oxygen. A still further requirement is that no by-products or residues are produced that may have an adverse biological effect on the patient.
As adults age the accommodative power of the eye decreases leading to the onset of presbyopia. This age-related decrease in accommodative power is believed to be caused by a decrease in the elasticity of the lens material. This decrease is probably caused by cross-linking of the lens material. Thus the loss of accommodation results from a change in elasticity rather than a decrease in the action of the ciliary muscles. The replacement of the original lens with a synthetic polymer having the elasticity equivalent to the lens of a young adult offers the prospect of being able to use a surgical procedure to replace the need for glasses to correct presbyopia.
U.S. Pat. No. 5,079,319 assigned to Ciba-Geigy Corporation discloses vinyl unsaturated macromonomers that are prepared via a two stage process. In the first stage of this process copolymers are prepared by addition polymerisation of ethylenically unsaturated monomers. The monomers are selected such that the polymer chain includes polysiloxane units pendant from a carbon backbone. Ethylenic unsaturation is introduced into the copolymer in the second stage by reaction of an active hydrogen in the polymer chain with an unsaturated isocyanate. The unsaturated macromonomer so formed may be subsequently crosslinked in a mould to form contact lenses. This invention is described as being an improvement over U.S. Pat. No. 4,605,712 which has a common assignee. The improvement is described as being the ability to introduce higher concentrations of siloxane groups without the problems of phase-separation and opacity associated with the compositions of U.S. Pat. No. 4,605,712. U.S. Pat. No. 5,079,319 asserts that addition polymerisation in the first stage leads to more random introduction of siloxane groups and thus the avoidance of large incompatible domains.
Silicones containing pendant (meth)acrylic or (meth)acrylamide groups are particularly suitable for rapid crosslinking using radical photoinitiating systems. U.S. Pat. No. 4,563,539 assigned to Dow Corning Corporation, discloses the synthesis of acrylofunctional siloxanes.
International Publication No. WO99/47185 in the name of Pharmacia & Upjohn Groningen BV proposes intraocular compositions comprising aqueous dispersions of polymerisable microgels. This application relies upon the microgel structure to obtain the rapid crosslinking required for intraocular lens procedures. The invention of this application is distinguished from this invention as it uses oligomers and solution polymers. The citation claims that it is not possible to achieve the required balance of speed of crosslinking, refractive index and mechanical properties necessary for intraocular procedures with such polymers.